"The immediate terror associated with the tsunamis and the earthquake itself may be dwarfed by the longer term suffering of the affected communities," said David Nabarro, the top official at the World Health Organisation dealing with humanitarian crises.

"There is a chance that we could have at least as many dying from communicable diseases as we had dying from the tsunami," he added as the world's largest aid operation rolled into several countries in the region,

"In each disaster the priority is to get water, sanitation, food and shelter to people and if we can do it promptly within the next two or three weeks then we will not have outbreaks of communicable diseases," Nabarro told journalists.

Nabarro said the fears were particularly great in the Indonesian region of Aceh, which accounted for at least 27,000 of the 55,000 deaths in south Asia according to estimates.

"For us the epicentre of our concern is the province of Aceh," Nabarro said, referring to the combination of damage to infrastructure and housing as well as a "weakened society" because of the internal conflict there.

Countries like Thailand, India, Sri Lanka and the Maldives had "excellent" public health systems to help them cope with the emergency, he added.

Aid agencies have warned about the threat of diarrhoeal diseases due to contaminated water, as well as the spread of mosquito-borne diseases such a dengue fever and malaria, which are already present in the region.

"By and large we find that the presence of a decaying human corpse is not generally a source of infection to the nearby population," Nabarro noted, repeating advice issued by the WHO after many disasters in recent years.

The UN, the Red Cross and Doctors Without Borders (Medecins Sans Frontieres), as well as local groups and neighbouring countries, were rushing aid to the most affected countries.

Emergency relief included shelter equipment and medical supplies, as well as specialists in water and sanitation.